Two things come to mind as you ask your question.
One is that our experience is that all throughout Markus's treatment, he was very involved, right from the first bad scan that showed that before the cancer had even been gone, it was back and into his lungs. Marcus was consulted. It was his decision to continue to proceed with treatment. It was his decision that day that my wife referenced when our oncologist said, “There's nothing more we can do.” It was his decision, along with us, to say, “I want treatment so that my quality of life is as good as it can be in the days that I have left.”
The other thought I had as you asked your question is that I want to emphasize the point that when you make it available to minors, then people like Markus and families like ours will be in a position where they're obliged to consider this. That doesn't sound like autonomy to me.